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Continuous Probability
Distributions
Chapter 7
McGraw-Hill/Irwin
Copyright © 2010 by The McGraw-Hill Companies, Inc. All rights reserved.
GOALS
1.
2.
3.
4.
5.
6.
7.
8.
7-2
Understand the difference between discrete and continuous
distributions.
Compute the mean and the standard deviation for a uniform
distribution.
Compute probabilities by using the uniform distribution.
List the characteristics of the normal probability distribution.
Define and calculate z values.
Determine the probability an observation is between two
points on a normal probability distribution.
Determine the probability an observation is above (or below)
a point on a normal probability distribution.
Use the normal probability distribution to approximate the
binomial distribution.
The Uniform Distribution
The uniform probability distribution is perhaps
the simplest distribution for a continuous
random variable.
This distribution is rectangular in shape and is
defined by minimum and maximum
values.
7-3
The Uniform Distribution - Example
EXAMPLE
Southwest Arizona State University
provides bus service to students
while they are on campus. A bus
arrives at the North Main Street
and College Drive stop every 30
minutes between 6 A.M. and 11
P.M. during weekdays. Students
arrive at the bus stop at random
times. The time that a student
waits is uniformly distributed from
0 to 30 minutes.
1.
Draw a graph of this distribution.
2.
Show that the area of this uniform
distribution is 1.00.
3.
How long will a student “typically”
have to wait for a bus? In other
words what is the mean waiting
time? What is the standard
deviation of the waiting times?
4.
7-4
What is the probability a student
will wait between 10 and 20
minutes?
P(10  Wait Time  20)  (height)(b ase)
1

(10)
(30  0)
 0.3333
Normal Probability Distribution
1.
2.
3.
4.
5.
6.
It is bell-shaped and has a single peak at the
center of the distribution.
It is symmetrical about the mean
It is asymptotic: The curve gets closer and closer
to the X-axis but never actually touches it.
The location of a normal distribution is
determined by the mean,, the dispersion or
spread of the distribution is determined by the
standard deviation,σ .
The arithmetic mean, median, and mode are
equal
The total area under the curve is 1.00; half the
area under the normal curve is to the right of this
center point and the other half to the left of it
Family of Distributions
Different Means and
Standard Deviations
Equal Means and
Different Standard
Deviations
Different Means and Equal Standard Deviations
7-5
The Standard Normal Probability Distribution




7-6
The standard normal distribution is a
normal distribution with a mean of 0 and
a standard deviation of 1.
It is also called the z distribution.
A z-value is the signed distance
between a selected value, designated X,
and the population mean , divided by
the population standard deviation, σ.
The formula is:
The Normal Distribution – Example
The weekly incomes of shift
foremen in the glass
industry follow the
normal probability
distribution with a mean
of $1,000 and a
standard deviation of
$100.
What is the z value for the
income, let’s call it X, of
a foreman who earns
$1,100 per week? For a
foreman who earns
$900 per week?
7-7
The Empirical Rule - Example
As part of its quality assurance
program, the Autolite Battery
Company conducts tests on
battery life. For a particular
D-cell alkaline battery, the
mean life is 19 hours. The
useful life of the battery
follows a normal distribution
with a standard deviation of
1.2 hours.
Answer the following questions.
1. About 68 percent of the
batteries failed between
what two values?
2. About 95 percent of the
batteries failed between
what two values?
3. Virtually all of the batteries
failed between what two
values?
7-8
Normal Distribution – Finding Probabilities
EXAMPLE
The mean weekly income of a shift
foreman in the glass industry is
normally distributed with a mean of
$1,000 and a standard deviation of
$100.
What is the likelihood of selecting a
foreman whose weekly income is
between $1,000 and $1,100?
7-9
Normal Distribution – Finding Probabilities
(Example 2)
Refer to the information regarding the weekly income
of shift foremen in the glass industry. The
distribution of weekly incomes follows the
normal probability distribution with a mean of
$1,000 and a standard deviation of $100.
What is the probability of selecting a shift foreman in
the glass industry whose income is:
Between $790 and $1,000?
7-10
What is the probability of selecting a shift foreman in the
glass industry whose income is:
Between $840 and $1,200
Using Z in Finding X Given Area - Example
Layton Tire and Rubber Company wishes to set a
minimum mileage guarantee on its new MX100
tire. Tests reveal the mean mileage is 67,900
with a standard deviation of 2,050 miles and
that the distribution of miles follows the normal
probability distribution. Layton wants to set the
minimum guaranteed mileage so that no more
than 4 percent of the tires will have to be
replaced.
What minimum guaranteed mileage should Layton
announce?
Solve X using the formula :
x -  x  67,900
z


2,050
The value of z is found using the 4% informatio n
The area between 67,900 and x is 0.4600, found by 0.5000 - 0.0400
Using Appendix B.1, the area closest to 0.4600 is 0.4599, which
gives a z alue of - 1.75. Then substituti ng into the equation :
- 1.75 
x - 67,900
, then solving for x
2,050
- 1.75(2,050)  x - 67,900
x  67,900 - 1.75(2,050)
x  64,312
7-11
Normal Approximation to the Binomial
7-12

The normal distribution (a continuous distribution) yields a good approximation of the binomial
distribution (a discrete distribution) for large values of n.

The normal probability distribution is generally a good approximation to the binomial probability
distribution when n and n(1- ) are both greater than 5. This is because as n increases, a
binomial distribution gets closer and closer to a normal distribution.
Continuity Correction Factor
The value .5 subtracted or added, depending on the
problem, to a selected value when a binomial
probability distribution (a discrete probability
distribution) is being approximated by a
continuous probability distribution (the normal
distribution).
Only one of four cases may arise:
1.
7-13
For the probability at least X occurs, use the
area above (X -.5).
2.
For the probability that more than X occurs,
use the area above (X+.5).
3.
For the probability that X or fewer occurs, use
the area below (X -.5).
4.
For the probability that fewer than X occurs,
use the area below (X+.5).
Normal Approximation to the Binomial - Example
Suppose the management of the Santoni Pizza Restaurant found that 70 percent of its new
customers return for another meal. For a week in which 80 new (first-time) customers
dined at Santoni’s, what is the probability that 60 or more will return for another meal?
7-14
P(X ≥ 60) = 0.063+0.048+ … + 0.001 = 0.197
Normal Approximation to the Binomial - Example
Suppose the management of the Santoni Pizza
Restaurant found that 70 percent of its new
customers return for another meal. For a
week in which 80 new (first-time) customers
dined at Santoni’s, what is the probability
that 60 or more will return for another meal?
Step 1. Find the mean and the variance of a
binomial distribution and find the z
corresponding to an X of 59.5 (x-.5, the
correction factor)
Step 2: Determine the area from 59.5 and
beyond
7-15